I've been pronouncing γαμμα as a "g" uniformly, but recently I've come across the notion that in the Koine period, it at certain times carried an English consonantal "y" sound. According to the reconstructed Koine system, when do I pronounce as "g" and when as "y"?

The ancient voiced stops β, δ, γ appear to have become fricatives first [IPA β, δ, γ], before the
ancient stops φ, θ, χ. Already at the beginning of the Roman period βῆτα was becoming a 'soft'
bilabial fricative, probably like Spanish sound of 'b' in Havana/Habana (the city). γάμμα became
a velar fricative, and even a palatal fricative in conjunction with front vowels. During the Roman
period δέλτα eventually became fricativized everywhere (like the English sound in 'this')18.
[p 223]

Likewise, notice the examples of γάμμα reflecting a fricative abound, in some cases approaching
a palatal [y] sound.20 The insertions and the substitutions with ι would not be probable without
γάμμα having become a soft fricative.